The measurement of the concentration of SO.sub.2 in gases is required not only for the purpose of preventing environmental pollution, but also provides important information for controlling the refining process of nonferrous metals, particularly smelting sulfides thereof. It has recently been found that the concentration of SO.sub.2 in a gas mixture can be determined by making use of a concentration cell in which a solid electrolyte of sulfates is used. The solid electrolytes used include potassium sulfate (K.sub.2 SO.sub.4) and sodium sulfate (Na.sub.2 SO.sub.4). Due to their poor heat resistance, durability and workability, such solid electrolytes can only be applied to gas sensors on a laboratory scale, and cannot be used practically for industrial applications such as smelting of nonferrous metals. Difficulties are also encountered in the industrial manufacturing of gas sensors from these electrolytes.